The overall objectives are (a) To isolate and characterize each of a series of extracellular cytolytic proteins of staphylococci and streptococci, with a view to studying their mechanisms of action and with a view to understanding their role in the genesis of disease. In addition to examining known products of microbial growth we aim also to discover new extracellular products that may be of significance in pathogenesis. (b) To explore the possible usefulness of cytolytic bacterial proteins as reagents for the elucidation of membrane structure. Emphasis will be placed on staphylococcal sphingomyelinase (beta- toxin), staphylococcal gamma-toxin, streptolysin O, and corynebacterial phospholipase D.